I. Field of the Invention
The present invention relates generally to numerical control systems and, more particularly, to a numerical control system for use with a table having a relatively stationary part and a movable part and which is capable of highly accurately positioning the movable and stationary parts with respect to each other.
II. Description of the Prior Art
There are a number of previously known numerical control systems for use with rotary tables having a relatively stationary part and a movable part. In such tables, a motor is operatively coupled between the movable and stationary parts and, upon actuation, sequentially positions the movable table part in a series of predetermined positions. A machining operation or the like is typically performed after the table has moved to each predetermined position.
In order to accurately position the table at each of its predetermined positions, it has been the previous practice to employ a position transducer operatively disposed between the table movable and stationary parts. Inductosyns and resolvers are two commonly used types of position transducers for such numerical control systems.
Although an inductosyn can be used in several different fashions, in one mode of operation a pair of sinusoidal waves are fed as input signals to the inductosyn and these input signals are offset by a predetermined phase difference, typically ninety (90.degree.) degrees. Relative movement of the table movable and stationary parts causes a like movement of the inductosyn which, in turn, shifts the phase of the output signal from the inductosyn which is also sinusoidal in form. With the inductosyns currently available a phase shift of 180 degrees of the output signal occurs for every degree of revolution of the inductosyn.
These previously known numerical control systems, however, are disadvantageous in that the movable table part could only be automatically positioned on even degree intervals of movement of the table which is not sufficiently accurate from many types of precision machining operations. If additional accuracy was required, it was the previous practice with many types of the previously known numerical control systems to manually actuate the table motor means while the table operator visually read a gauge indicative of the table position between even degree increments of the inductosyn. Since automatic position control for the table was not possible when high accuracy positioning of the table was required, the overall operation of these previously known systems was relatively slow.
In still other types of previously known numerical control systems, however, automatic positioning of the table between even degree increments of the inductosyn was possible. However, even with this latter type of previously known numerical control system, the accuracy of the table position control was still limited to relatively large increments, for example 1/10 of one degree of rotation, which is still unsuitable for some high precision machining operations. Moreover, the accuracy of many of these previously known systems was dependent upon the accuracy and stability of the system clock.